It is common to utilize microstrip patch antennas in environments where a planar antenna is required. In situations that require dual band antennas, dual band microstrip patch antennas may be based on slotted patches, stacked parasitic patches, or by introducing certain reactive loadings into the structure. A uniplanar structure is usually preferred as it eases the fabrication process compared with other dual band solutions, such as a vertically stacked parasitic patch antennas. However, it is difficult to design multiband uniplanar microstrip antennas as the two microstrip radiators have to be printed on the same side of a substrate. If two rectangular (or circular) patches are used each corresponds to a different frequency and need to be placed side-by-side. This placement may generate several noted problems including, for example, occupying a large area. A further noted problem is that the two patches have different phase centers. Further, the two patches have strong couplings which reduces the gain and may further degrade the axial ratio for CP antennas.
Another prior art design is to utilize a concentric microstrip ring that surrounds a second patch center. However, this design also includes several noted disadvantages including the fact that the concentric ring has to resonate at TM11 mode, which is generally difficult to be matched to 50 ohms. Further, the radiation comes from both edges of the ring, thereby causing increased interaction with the inner radiator. Further, the surface wave bouncing inside the substrate further increases coupling between the radiators and feeds. As is known by those skilled in the art, the bandwidth of microstrip antennas is proportional to the substrate thickness and is inversely proportional to its permittivity. Antennas on thin substrates suffer from high dielectric/conductor losses. Therefore, thick substrates are generally utilized in such applications. However, the antenna efficiency decreases while thickness increases since the non-cut-off surface wave, which is generally TM0 mode wave, is prone to be excited and propagate along the grounded substrate. This wastes power as heat.